My preference is as close as poss to the UFH manifold, allows everything for the UFH to be held together. A lot of the UFH valves are also run from the UFH control centre and that is usually close to the UFH manifold too to minimises cable runs but it all depends on locations of each item.
That being said I have to agree with @Exedon, in the grand scheme of things and how the system works it doesn't really matter where it's located, as long as it's clearly marked if it's not apparent. Same with all the other valves.